This invention generally pertains to injection molding. More specifically, the present invention relates to a process and apparatus for injection molding under a high pressure and moldings produced thereby.
The invention is particularly applicable to the injection of a gas under high pressure into a molten plastic stream as it enters a mold sprue and a mold space. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments.
It has become known that it is advantageous to urge the molten plastic material in an injection mold outwardly into contact with the mold surfaces by exerting pressure on the molten plastic. This aids the external surface of the plastic material in assuming the precise shape dictated by the mold surfaces. The pressure also assists the filling of the mold space with molten plastic even if the space is elongated or narrow and is normally difficult to fill. Such pressure can be applied by a fluid which is injected into the plastic material in the mold space. This is advantageous since the molded part produced utilizes less plastic material and is lighter than if the part were solid plastic.
Heretofore, conventional injection molding apparatuses have attempted to simultaneously inject a pressurized fluid and a molten plastic material into a mold cavity. Difficulties have, however, been experienced when the plastic needs to be injected at a high pressure since then a high pressure fluid (preferably gas at 4,000 to 15,000 psi) is also required. Conventional injection molding apparatuses utilize pump arrangements, such as a piston and cylinder pump, for pressurizing the gas when its injection into the molten stream of plastic material is called for. Unfortunately, the response time of a conventional piston and cylinder arrangement for pressurizing the gas to a suitably high pressure is approximately two to three seconds. Usually, however, the injection molding process itself is completed in these two to three seconds so that by the time the piston and cylinder arrangement has pressurized the gas to an adequate pressure to enter the mold space (for example, 9,000 psi) the injection molding of the molten plastic is completed. At this point, the gas will explode into the mold space and pressurize the plastic part that has been molded and move toward the surfaces of the mold cavity whatever molten plastic remains therein.
It should be noted that gas at high pressure, in the range of generally 4,000 to 15,000 psi, is unavailable from pressurized cylinders or the like that are commercially available. Although it is true that a 6,000 psi gas cylinder has now become available in certain areas, generally only gas cylinders pressurized at 2,500 psi are available. Thus gas from a conventional supply source needs to be pressurized before it can be utilized in an injection molding process. If such pressurization is not done before the injection molding process begins, the gas will generally only be adequately pressurized to its injection pressure after the molten plastic has already been injected.
To take one example, if a mold space is 90% full of molten and partially solidified plastic, and if there are a lot of bosses, ridges and ribs in the part to be molded then, before the pressurized gas enters the mold cavity, there will be numerous bad sink marks in the molded product. When the pressurized gas is injected after the injection of the molten plastic, the gas will urge the plastic outwardly and pushed the sink marks out against the surfaces of the mold space. This, however, causes shadow marks on the plastic product. Such shadow marks are very evident and are unacceptable for a class A finish. Moreover, gas entering the mold space after the plastic material has essentially stopped flowing will push the plastic more strongly in its thinner zones than in its thicker zones, as would be expected. In other words, the gas will drive the plastic somewhat sideways as well as outwardly. Additionally, the extremities of a plastic product will still get sink marks.
If, on the other hand, the product is not full of ribs, ridges, and bosses, then, when the plastic originally stops flowing, there will be a nonfilled area in the mold space. When the gas enters the mold space, it will push whatever molten plastic remains into the nonfilled areas so that the plastic will coat all the surfaces of the mold space. However, a clear line of demarcation will be evident on the surface of the product to show where the gas has compelled the plastic to flow again after it had initially stopped. This, again, is unacceptable for a class A finish. In both of the above instances post molding treatment, such as painting, is called for and this, obviously, adds to the expense of the molded part.
Accordingly, it has been considered desirable to develop a new and improved method for injection molding and an apparatus therefor and moldings produced thereby which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.